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Block IPs that Don't Resolve via Reverse DNS Lookup

I'm sure I'm not the only one who's tried to do this, but I've checked the forums and Google and couldn't find any instructions or documentation on this one: is there any way to tell Zimbra to block email from any IP address that doesn't resolve via a reverse DNS lookup?

That's probably what I want, but I'm not entirely familiar with how the "reject_unknown_sender_domain" command works, so I want to make sure this will fulfill my original request before I implement it.